Overcoming flexible substrate limitations: Dual optimization of alkali doping and compositional grading in high efficient CIGS photovoltaics

This study presents a systematic investigation of high-efficiency flexible copper indium gallium selenide (CIGS) thin-film solar cells fabricated using an optimized three-stage co-evaporation process. The research focuses on two key innovations: (1) NaF pre-deposition for controlled alkali metal doping and (2) active regulation of In/Ga evaporation timing during the initial growth stage to precisely engineer the Ga/(Ga+In) (GGI) ratio gradient throughout the absorber layer depth. Through comprehensive characterization of structural properties, elemental distributions, and device performance, we demonstrate that the synergistic combination of Na doping and tailored Ga grading effectively addresses critical challenges in flexible CIGS devices, including back-surface Ga accumulation and non-ideal bandgap profiles. Our results reveal that this dual optimization strategy significantly enhances charge carrier mobility and collection efficiency, ultimately leading to substantial improvements in overall solar cell performance. The findings establish a robust materials engineering approach for developing high-performance flexible photovoltaic devices through precise control of compositional gradients and defect passivation.